Equipment for producing a three-dimensional object layer by layer using a powdery material which can be solidified, or fused together, by irradiating it with a high-energy beam of electromagnetic radiation or electrons are known from e.g. U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,647,931 and SE524467. Such equipment include for instance a supply of powder, means for applying a layer of powder on a vertically adjustable platform or working area, and means for directing the beam over the working area. The powder sinters or melts and solidifies as the beam moves over the working area.
When melting or sintering a powder using a high-energy beam, it is important to avoid exceeding the vaporization temperature of the powder, since otherwise the powder will merely vaporise instead of forming the intended product. US2005/0186538 discloses a method focusing on this problem. In this method a laser beam repeatedly is directed to the same powder target area during the melting/sintering phase as to stepwise raise the powder temperature. This way, a too high powder temperature is avoided.
When using an electron beam instead of a laser beam, the situation is in some ways different. As the electron beam hits the powder, a charge distribution develops around the electron target area. Desirably, this charge will be led through a produced part of the article to be made and/or the powder bed towards ground. If the charge distribution density exceeds a critical limit, an electrical field having a field strength above a predetermined level will develop around the position where the beam is radiating. The electrical field having an electrical field strength above the predetermined level will be referred to as Emax. An electrical field will cause the powder particles to repel each other such that particles leave the uppermost surface layer of the particle and create a distribution of particles floating above the surface. The floating particles resemble a cloud positioned above the surface. When the electrical field has a field strength above Emax, the electrical field, i.e. the particle cloud, will influence the resolution of the device in a negative way. This is partly due to the fact that the particles in the particle cloud will diverge the electron beam. When the electrical field has a field strength below Emax, the electrical field, i.e. the particle cloud, will not influence the resolution of the device in a significant way. A field strength below Emax is thus desirable.
Since the particles are charged they will seek a ground contact and thereby some may leave the cloud and will then contaminate different parts of the device being positioned inside the vacuum chamber. A result of such a critical electrical field is that the structure of the powder surface will be destroyed. Applying the method according to US2005/0186538 to a powder melting/sintering device equipped with an electron beam is likely to give a poor result since no measures are taken in that method to avoid a critical electrical having a field strength above said predetermined level to develop.
One solution to the problem of avoiding discharges is to add conductive material, such as carbon, to the powder as to increase the electrical conductivity of the powder. Disadvantages of this solution are, however, that the solidifying process of such a powder mixture may be difficult to control and that the properties of the formed product may be affected in a negative way. For instance, the mechanical strength may be decreased.